1. Field of the Invention
The present invention relates to an in-vehicle electronic control unit that starts an operation when a manual power supply switch, which, for example, an ignition switch, is closed, and more particularly, to improvements of a power feeding control method in power feeding control circuitry in which a power supply switch forms a power feeding circuit for a part of in-vehicle electrical loads.
2. Description of the Background Art
In order to prevent a load current from flowing when a power supply is connected in reversed polarity by error, a technique put in wide practical use is to connect a reverse connection protective diode in series to an electrical load which is fed with power from a DC power supply. A voltage drop and a temperature rise caused by a reverse connection protective element during a normal operation are suppressed by employing a field-effect transistor as the reverse connection protective element in a broad range of applications.
The field-effect transistor referred to herein can be either a P-channel type or an N-channel type. In either case, when the power supply is in proper polarity, the field-effect transistor is driven to close so that a current flows in a direction same as an energization direction of an internal parasitic diode.
For example, according to FIG. 1 of Patent Literature 1 specified below under the title of “protective apparatus of electronic device”, a protective apparatus 1 of an electronic device operating on a DC power supply is provided with a P-channel FET 3 for protection against reverse connection of the power supply. A drain of the P-channel FET 3 is connected to a power-supply terminal 2 on a positive electrode side, a source is connected to a power-supply input terminal of an electronic device 4, and a gate is connected to a grounding line.
By switching the P-channel FET 3 to an OFF state when the power supply is connected in reversed polarity and by making the ground common between a circuit 4a of the electronic device 4 and the power supply, a configuration that not only protects the circuit when the power supply is connected in reversed polarity, but is also resistant to noise can be achieved.
Regarding the terminals 2 and 2′ receiving a supply of power from the unillustrated DC power supply in the protective apparatus 1, the terminal 2 is a power-supply terminal on the positive electrode side and the terminal 2′ is an earth (or GND (ground)) terminal. For example, in the case of a device for vehicle, a voltage from the battery is supplied to the device when an unillustrated ignition switch is switched ON.
According to FIG. 4 of Patent Literature 2 specified below under the title of “power supply reverse connection protective circuit”, an ECU 45 operating on power of a battery 3 is configured as follows. That is, an N-channel FET 21 is provided on a power supply wire 15 linking a power supply terminal 5 connected to a plus terminal of the battery 3 and a control circuit 13 to be fed with power in such a manner that an anode of its parasitic diode D1 is on the side of the power supply terminal 5. Further, an N-channel FET 22 is provided downstream of the FET 21 in such a manner that a cathode of its parasitic diode D2 is on the side of the FET 21. When an ignition key switch 9 is switched ON while the battery 3 is connected in proper polarity, the FETs 21 and 22 are switched ON by charge pump circuits 43 and 47, respectively, to which operating power is supplied from the drain side of the FET 21. Power of the battery 3 is thus supplied to the control circuit 13. Also, when the battery 3 is connected in reversed polarity, the FETs 21 and 22 are switched OFF. A reverse current is thus prevented by the parasitic diode D1.
The control circuit 13 starts an operation upon receipt of power from the battery 3 when the FET 22 is switched ON. Once the control circuit 13 starts an operation, the control circuit 13 outputs a drive signal Sd to keep the FETs 21 and 22 switched ON even when the ignition key switch 9 is switched OFF. Although it is not shown in the drawing, the control circuit 13 monitors a voltage at a signal input terminal 11 to detect ON and OFF states of the ignition key switch 9. The control circuit 13 detects that the ignition key switch 9 is switched OFF on the basis of a voltage at the signal input terminal 11. Further, when the conditions to stop the operation are satisfied later by finishing pre-processing to stop the operation, such as data saving, the control circuit 13 stops outputting the drive signal Sd.